1. Field of the Invention
The present invention relates to a thin-film resistor used for various miniature electronic circuits and to a method for manufacturing the resistor.
2. Description of the Related Art
FIG. 7 is a plan view of a known thin-film resistor, FIG. 8 is a sectional view of the thin-film resistor, and FIGS. 9A to 9D are schematic drawings showing a process of the thin-film resistor. As shown in FIGS. 7 and 8, the known thin-film resistor comprises a resistive element 11 and a pair of electrodes 12 disposed on an alumina substrate 10. The resistance of the thin-film resistor is defined by the length L and the width W of the resistive element 11 between the electrodes 12.
In order to prepare the thin-film resistor having the above-described structure, first, TaN for the resistive element 11 and Al for the electrodes 12 are formed into films, in that order, on the alumina substrate 10 by vapor deposition, ion beam sputtering, or the like, as shown in FIG. 9A. Then the films are patterned into predetermined shapes by etching, ion milling, or the like. Next, as shown in FIG. 9B, the Al is covered with a photoresist by spin coating, and is subsequently exposed to light to form a resist pattern 13 having a predetermined shape. The Al exposed at the resist pattern 13 is subjected to wet etching, as shown in FIG. 9C. Thus, the thin-film resistor having the resistive element 11 between the electrodes 12 is completed, as shown in FIG. 9D.
The resistance of the electrodes 12 must be reduced in known thin-film resistors. However, the electrodes 12 are formed of an electrode material, such as Al, to a small thickness of about 100 to 500 nm by vapor deposition, ion beam sputtering, or the like, and therefore, it is difficult to sufficiently increase the thickness of the electrodes 12 and, consequently, to reduce the resistance. Also, patterning the electrode material by wet-etching to form the electrodes 12 causes a large amount of side etch in edges of the electrodes 12, as shown in FIG. 9C. As a result, the length L of the resistive element 11 between the electrodes 12 varies and thus the precision of the resistance is degraded. Instead of forming the single-layer Al electrodes, Cr/Cu, Cr/Cu/Cr, Cr/Au, Cr/Au/Cr, and the like can be used to form two-layer or three-layer electrodes. This multilayer structure causes stepped side etch in edges of the electrodes because the plurality of layers are subjected to wet etching to pattern the electrodes, thereby degrading the precision of the resistance, as in the single-layer electrodes.
Accordingly, an object of the present invention is to provide an accurate thin-film resistor which includes electrodes having a reduced resistance and which exhibits only a small range of variation in resistance.
To this end, according to one aspect of the present invention, a thin-film resistor is provided. The thin-film resistor has a substrate, a resistive element deposited on the substrate, and a tapered insulator layer patterned so as to cross over the resistive element in the width direction. A plating base layer is formed on the resistive element and the insulator layer and is divided into a pair of portions on the insulator layer such that the gap between the portions extends across the width of the resistive element. A pair of electrodes is formed on the surfaces of the pair of portions.
The present invention is also directed to a method for manufacturing a thin-film resistor including the steps of: depositing a resistive element having a predetermined length and width on a substrate; forming an insulating resist pattern defining an insulator layer on the substrate so as to cover all of the resistive element except the ends in the longitudinal direction of the resistive element; tapering the insulating resist pattern to form the insulator layer; forming a plating base layer on the substrate by plating to cover the resistive element and the insulator layer; forming a pair of electrodes on the surface of the plating base layer by plating such that the gap between the electrodes extends across the width of the resistive element; and removing the plating base layer between the electrodes.
By forming the electrodes to large thickness by plating, the resistance of the electrodes can be reduced. Also, since the resistance of the thin-film resistor is defined by the shape of the insulating resist pattern of the insulator layer, the resulting thin-film resistor can have high accuracy and a small range of variation of the resistance.
In the method for manufacturing the thin-film resistor, the step of tapering the insulating resist pattern may include a sub step of post-baking the insulating resist pattern and subsequently curing the insulating resist pattern. Preferably, after post baking, the insulating resist pattern is exposed to ultraviolet light and is then cured. By being exposed to ultraviolet light, the original shape of the tapered insulating resist pattern formed by post baking can be maintained even after curing.